1. Field of the Invention
The present invention relates to integrated-circuit package designs and, more specifically, to improved molded-plastic package designs for thermally enhanced integrated circuits.
2. Prior Art
FIG. 1A shows a conventional quad fiat package assembly 10 for an integrated-circuit die 12. The integrated-circuit die 12 is attached to an upset die-attach paddle portion 14, which is at the center region of a conventional lead frame 16. Various inwardly-extending leads terminate at their inner ends in bonding fingers (typically shown as 18 and 20). A layer of adhesive, not shown, is commonly used to attach die 12 and leads 18 and 20 to die-attach paddle 14. The bonding fingers are connected to respective bonding pads on the integrated-circuit die 12 using respective bonding wires (typically shown as 22), as indicated in the Figure. The entire assembly described above is conventionally encapsulated in a molded plastic material, which forms a molded-plastic body 26 for the package assembly 10.
FIG. 1B is a plan view of the lead frame 16 for the conventional quad fiat package assembly 10. Note that the ends of the bonding fingers do not extend all the way to the die-attach paddle and are not connected to the die attach paddle portion 14. This provides substantial spaces 30, 32, 34, and 36 between the inner ends of the bonding fingers 18 and 20 and the die attach paddle 14 for the flow of compound during the encapsulation process. The die attach paddle portion 14 is secured in place at the center of the lead frame by tie bars 17. The tie bars 17 extend inwardly from the corners of the lead frame 16 and, unlike bonding fingers 18 and 20, are attached to die paddle portion 14. The assembled die 14 and lead frame 16 combination is encapsulated in molded plastic material by being placed in a cavity formed by the two halves of a mold and by injecting plastic material into the top half of the mold at one corner of the lead frame. Air vents are provided in the mold at the other three corners of the package. Some of the paths for the plastic material to flow into the bottom half of the mold from the top half of the mold are provided by the spaces 30, 32, 34, and 36 which are present between the inner ends of the bonding fingers and the die-attach paddle. Other paths are provided by the spaces between the leads of the lead frame 16. These flow paths permit the flow of the plastic molding material top be substantially balanced between the top half and the bottom half of the of the mold. As the plastic material flows through the mold, air is expelled out of the air vents at the three corners of the mold by the flowing plastic material so that no air remains trapped within the molded-plastic body. If air were to be trapped, it would cause voids, blow holes, or pin holes, in the molded plastic body 26.
FIG. 2 shows a package mold 60, which has a top mold-half 62 and a bottom mold-half 64, for molding a thermally-enhanced, quad fiat package. A thermally-enhanced, electrically-insulated substrate 66, which is formed of a material such as, for example, aluminum nitride, has an integrated-circuit die 68 mounted thereto. The thermally conductive, electrically-insulated substrate 66 replaces a conventional die attach paddle (such as the die-attach paddle 14 of FIGS. 1A and 1B) and improves the thermal performance of a molded-plastic package. Bonding fingers (typically shown as 69 and 70) at the inner ends of the leads of a lead frame 72 are attached to the outer margins of the thermally conductive, electrically-insulated substrate 66 with an adhesive film 73 formed of a material such as R-flex 1000. A layer of adhesive, not shown, is also commonly used to attach die 68 to substrate 66. As in the case of a conventional quad fiat package assembly, the thermally conductive, electrically-insulated substrate 66 and its attached integrated-circuit die 68 are placed in the cavity formed between the two halves 62 and 64 of the mold 60. Plastic material is injected into the top half of the mold at the inlet gate 74. The plastic material enters the top half 62 of the mold and flows through the spaces between the bonding fingers of the lead frame into the bottom half of the mold. Vents 76 in the mold corners release trapped air.
The arrows shown in FIG. 2 indicate the flow of plastic molding material through the top half of the mold and through the bottom half of the mold. Note that the thermally conductive, electrically-insulated substrate 66 is much greater in thickness and is also much wider than the conventional die-attach paddle 14 shown in FIGS. 1A and 1B. The intrusion of the much thicker bulk of the thermally conductive, electrically-insulated substrate 66 disrupts and unbalances the flow of plastic material in the mold in several ways.
One way that flow is disrupted is that the open spaces between the ends of the bonding fingers and the edge of the integrated-circuit die are blocked by the substrate 66.
Another way that flow is disrupted is that the bulk of the substrate intrudes into the lower half of the mold so that the cross-sectional area for the flow of molding material in the lower space of the cavity is smaller and the flow resistance is greater for the lower space. This results in the flow of the molding material in the upper half of the mold being faster than the flow of molding material in the lower half of the mold. As a result of the differences in flow, the air at different places within the mold halves is expelled at different rates so that, for example, some air is trapped within the bottom part of the mold. The trapped air creates voids, also called blow holes or pin holes, in the body of the package. A typical void 80 is created on the side of the package which is opposite the inlet gate 74, as illustrated in FIG. 2.
FIG. 3 shows a package mold 60, as shown in FIG. 2, having a top mold-half 62 and a bottom mold-half 64, for molding a thermally-enhanced, quad fiat package. However, instead of the thermally-enhanced, electrically-insulated substrate 66 of FIG. 2, a slug 82 formed of copper or other similar metal is used as the substrate to which integrated-circuit die 68 is mounted. A layer 84 of insulating material electrically insulates the bottom of the integrated circuit die 68 from the top surface of the slug 82. The slug 82 replaces a conventional die attach paddle (such as the die-attach paddle 14 of FIGS. 1A and 1B) and improves the thermal performance of a molded-plastic package. Bonding fingers (typically shown as 69 and 70) at the inner ends of the leads of a lead frame 72 are attached to the outer margins of the slug 82 with an insulating layer of material disposed between the ends of the bonding fingers 69 and 70 and the slug 82 to insure that the slug 82 is electrically insulated from the bonding fingers 69 and 70 of the lead frame 72.
However, as indicated in FIG. 3, the bottom portion of the slug 82 may extend completely to the bottom of the mold cavity. Thus, after encapsulation of the entire assembly described above, the bottom of slug 82 remains exposed so that heat may be conducted away from integrated-circuit die 68. Since slug 82 extends to the bottom of the mold cavity, the plastic encapsulating material in the bottom of the mold cavity is unable to flow under slug 82 and is only able flow around the edges of slug 82. Again, this results in the flow of the molding material in the upper half of the mold being greater than the flow of molding material in the lower half of the mold, and as a result of the differences in flow, air within the mold halves is expelled by plastic material flowing at different rates creating voids or blow holes in the body of the package.
Consequently, a need exists for a technique to prevent the formation of voids on the body of a thermally-enhanced molded plastic package body caused by differences in the flow rate of the encapsulating material through the mold.